This invention relates to the preparation of polyesters, and more particularly to the preparation of poly(1,4-cyclohexanedicarboxylate)s.
Poly(1,4-cyclohexanedicarboxylate)s such as poly(1,4-cyclohexanedimethylene-1,4-cyclohexanedicarboxylate), hereinafter sometimes designated xe2x80x9cPCCDxe2x80x9d for brevity, are known polyesters; reference is made, for example, to U.S. Pat. No. 2,891,930. PCCD is characterized by such advantageous properties as crystallinity and resistance to weathering under conditions of exposure to ultraviolet radiation. These properties have increased interest in commercialization in recent years.
Commercially employed methods of producing PCCD utilize as reactants at least one relatively non-volatile diol, most often 1,4-cyclohexanedimethanol (hereinafter xe2x80x9cCHDMxe2x80x9d) and at least one dialkyl 1,4-cyclohexanedicarboxylate, most often dimethyl 1,4-cyclohexanedicarboxylate (hereinafter xe2x80x9cDMCDxe2x80x9d). Polymer formation proceeds with the initial formation of oligomers containing ester and hydroxy end-groups, which react together to build molecular weight. The intermediate mixtures of diester, diol and oligomers are miscible in all proportions, forming a clear melt at an early stage of the polymerization reaction. U.S. Pat. No. 6,084,055 is an example of such transesterification procedures.
The DMCD used in this process is ordinarily obtained by reduction of dimethyl terephthalate. Its production, therefore, requires two steps: esterification of terephthalic acid and reduction to form DMCD. It would be desirable to employ cyclohexane-1,4-dicarboxylic acid (hereinafter xe2x80x9cCHDAxe2x80x9d), which can be obtained in one step by reduction of terephthalic acid, as a reactant in the preparation of PCCD.
The employment of CHDA is not as easy, however, with relatively non-volatile diols such as CHDM as with more volatile ones such as ethylene glycol and 1,4-butanediol. Such volatile diols can be used in excess, driving the esterification to completion, and the excess can then be removed by distillation. Any excess of relatively non-volatile CHDM will, by contrast, remain in the polyester as a contaminant, rendering the polyester useless or at least materially degrading its properties. Another common issue with employing non-volatile CHDM in excess is that the molecular weight of the resulting polyester is limited. Moreover, mixtures of CHDM, CHDA and polyester oligomers are not uniformly miscible since CHDA is not highly soluble in CHDM.
Thus, those attempting to prepare PCCD from CHDM and CHDA find themselves on the horns of a dilemma. If they attempt to employ a large excess of CHDM to maximize the molecular weight of the product, they obtain a polyester that may be essentially useless, for example owing to its contamination with excess CHDM. If, on the other hand, they attempt to employ the reagents in essentially or nearly equimolar proportions, molecular weight control may be a problem since production of high molecular weight polyester requires very close and consistent stoichiometry matching of reactants.
Various factors can affect the crystallinity of PCCD and similar polyesters. One of the major ones is the cis/trans isomer ratio of the diester moiety, especially the acid-derived moieties therein. CHDM and CHDA exist in cis and trans isomeric forms, and the degree of crystallinity of the polymer decreases significantly with an increase in proportion of cis isomer content. For the most part, the reagents DMCD and CHDA have a trans isomer level of 99% by weight or greater. (CHDM is about 70% trans but no isomerization of the diol moieties occurs during polymerization.) The proportion of trans isomer in the polyester itself is lower, in one embodiment about 87% or greater for optimum crystallinity.
Therefore, a method of making high molecular weight, highly crystalline poly(1,4-cyclohexanedicarboxylate)s in essentially contaminant-free form is desirable. In a particular embodiment a method of making high molecular weight, highly crystalline poly(1,4-cyclohexanedicarboxylate)s substantially free from cyclohexane-1,4-dicarboxylic acids is desirable. Such a method is provided by the present invention.
The present invention is based on the discovery that mixtures of diols comprising CHDM with CHDA and oligomers thereof, may be employed to make high molecular weight polyesters under suitable conditions wherein, as the polymerization proceeds and CHDA becomes incorporated into the polymer, a clear melt is formed. This can be accomplished without resorting to temperatures so high as to cause conversion in the monomers or polymer product of the trans to the cis isomer.
The invention, therefore, is in one embodiment a method for preparing a poly(1,4-cyclohexanedicarboxylate) which comprises bringing into mutual contact, in the presence of an esterification catalyst, reagents comprising at least one relatively non-volatile diol and cyclohexane-1,4-dicarboxylic acid, the molar ratio of total diol to acid being in the range of 0.97-1.02:1, said contact taking place at a temperature in the range of 220-260xc2x0 C.